1. Field of the Invention
The invention relates to a communication environment with a plurality of radio systems which have at least one radio station. Before a useful signal is transmitted, the radio station checks whether a certain sub-frequency range in a predefined frequency range is free.
2. Description of the Related Art
From the document "R. O. LaMaire and A. Krishna: On the Advantages and Disadvantages of Some Options for Sharing Etiquettes in the 5.15-5.35 GHz band, 5 GHz Sharing Rules Drafting Committee, SRDC/10.28.96.13B, Nov. 14, 1996" is known a radio system which works in the 5 GHz band according to the LBT method (LBT=Listen Before Talk). Radio stations implementing, for example, a CDMA or TDMA method may occupy a part of a predefined spectrum or of a frequency range. First of all it is then necessary for a radio station to check whether a sub-frequency range is free. This is determined by means of an energy detection in the desired sub-frequency range. If the radio station detects that the certain sub-frequency range is free and thus no other radio station utilizes the certain sub-frequency range for signal transmission, this sub-frequency range is; seized for transmitting useful signals. However, it may happen that two radio stations simultaneously seize the certain sub-frequency range. In that case a proper useful signal transmission cannot take place due to the simultaneous seizure of the sub-frequency range by, for example, two radio stations.